HP 7470A Interfacing And Programming Manual page 113

)
}
)
)
)
)
)
)
)
)
)
The following flow diagram illustrates the functional elements o
typical software checking handshake within a user's program.
START
PREPARE BLOCK
OF DATA FOR TRANS-
MISSION TO PLOTTER
SEND OUTPUT
BUFFER COMMAND
ESC.B TO PLOTTER
SEND DATA
BLOCK TO PLOTTER
>-18 RS-232-C/CCITT V.24 INTERFACING
Xon-Xoff Handshake
With the Xon-Xoff handshake method, the plotter controls the data
exchange sequence by telling the computer when it h a s room in its
buffer for data and when to shut off the flow. The plotter uses buffer
threshold indicators (an Xon trigger character and an Xoff trigger
character) to prevent buffer overflow.
OVERSHOOT (DUE TO TIME REQUIRED TO
REACT TO XOFF TRIGGER CHARACTER!
BUFFER FULL
XOFF THRESHOLD
(XOFF TRIGGER
CHARACTER SENT)
XON THRESHOLD
(XON TRIGGER
CHARACTER SENT)
BUFFER EMPTY
TOTAL
BUFFER
SPACE
AVAILABLE
2S6
Xon-Xoff Threshold Levels
As data is sent to the plotter by the computer, it is stored in the buffer
and simultaneously acted on by the plotter. The preceding figure is
representative of the way the Xon-Xoff handshake works; the numbers
represent the following:
1. Data enters the buffer faster than it can be acted on by the plotter,
and the buffer starts to fill.
2. The plotter begins processing the input data faster than the computer
sends it, and the buffer starts to empty.
3. The data enters the buffer at a faster rate than the plotter can
process it. The amount of data stored in the buffer reaches the Xoff
threshold level, at which point the plotter sends the Xoff trigger
character stopping the flow of data from the computer.
4. Due to a finite delay between the time the plotter sends the Xoff
trigger character and the time it takes the computer to react, a slight
overshoot may occur. For this reason, the Xoff threshold level should
always be specified at least as large as the data block size or the
RS-232-C/CCITT V.24 INTERFACING 10-19